Feedback stabilization of a magnetically confined plasma

ABSTRACT

A high-temperature plasma is produced by the injection and trapping of energetic particles in a magnetic field containment zone. Detectors are arranged proximate to the plasma to detect electrostatic or electromagnetic signals produced by instabilities in the plasma containment system. The detected signals are amplified and shifted in phase either positively or negatively with respect to the instability oscillation and are then utilized to modulate an energetic particle beam which is then directed into the plasma containment zone wherein the modulation component of the injected beam interacts with the plasma to minimize or eliminate the instability oscillation of the plasma. Stable containment of the plasma is thereby promoted.

United States Patent FEEDBACK STABILIZATION OF A MAGNETICALLY CONFINEDPLASMA 10 Claims, 2 Drawing Figs.

U.S. Cl. 176/7, 315/111,3l3/l6l Int. Cl G2lb 1/00 Field of Search 176/1-9; 315/31 1; 313/161 References Cited UNITED STATES PATENTS 4/1962Warnecke et al Primary Examiner- Reuben Epstein Attorney-Roland A.Anderson ABSTRACT: A high-temperature plasma is produced by theinjection and trapping of energetic particles in a magnetic fieldcontainment zone. Detectors are arranged proximate to the plasma todetect electrostatic or electromagnetic signals produced byinstabilities in the plasma containment system. The detected signals areamplified and shifted in phase either positively or negatively withrespect to the instability oscillation and are then utilized to modulatean energetic particle beam which is then directed into the plasmacontainment zone wherein the modulation component of the injected beaminteracts with the plasma to minimize or eliminate the instabilityoscillation of the plasma. Stable containment of the plasma is therebypromoted.

PATENTEUuuvaomn 3.624.240

PHASE SHIFTERT POWER AMPLIFIER BEAM NEUTRALIZER INVENTORS. Charles C.Damm y Richard F Post ATTORNEY.

FEEDBACK STABILIZATION OF A MAGNETICALLY CONFINED PLASMA BACKGROUND OFTHE INVENTION The invention described herein was conceived, or made,under Contract NO. W-7405-ENG-48 with the United States Atomic EnergyCommission.

A variety of technological fields are concerned with the production ofhigh temperature gases or plasmas produced and contained in magneticcontainment zones having a wide variety of configurations. For example,the configurations may be toroidal, cylindrical or generally spherical.Such configurations are typified, in the controlled fusion or controlledthermonuclear reactor art by so-called stellarator, Astron, MagneticMirror, Levitron, ALICE, Baseball or Tennis Ball Scam and other types ofmachines or devices.

It is well known that the times of containment, plasma density,temperatures of the plasma and combinations thereof which areattainable, in practice, are usually limited by the onset of so-calledinstabiliities", i.e., disruptive oscillations of the plasma. Theseinstabilities may lead to a sudden loss or to an accelerated loss ofplasma from the containment zone far in excess of that predicted byclassical diffusion theory.

The occurrence of such instabilities has accordingly proven to be aserious obstacle in obtaining plasmas containing fusionable lightisotope ions at densities and temperatures and for periods of timesufficient to yield high and/or sustained thermonuclear reaction rates,e.g., such as may be employed in power producing controlled fusionreactors.

Various approaches have been utilized in attempting to eliminate orminimize the occurrence of instabilities in such magnetically containedplasmas such as employing magnetic fields with favorable linecurvatures, minimum-B configurations, stringently controlled operatingparameters and the like. One method wherein higher atomic weight plasmaparticle additives are injected into the plasma containment zone toeffect particle orbit stabilization of the plasma is disclosed in US.Pat. No. 3,257,284, issued June 21, l966 to Richard F. Post. Generalinformation relating to plasma instability is also disclosed in saidpatent. See also, Cooperative Oscillations in a High-Temperature PlasmaFormed by Neutral Atom Injection," R. F. Post et al., pp. 323-326, Vol.10, No. 8, Physical Review Letters, Apr. 15, l963. Also Observations ofPlasma Instability with Rotational Effects in a Mirror Machine, W. A.Perkins and R. F. Post, pp. 1537-1558, Vol. 6, No. ll, Physics ofFluids," Nov. 1963. More recent literature is replete with reports ofexperimentation and theoretical discussion directed to the problem ofcontrolling or eliminating instabilities from high temperature plasmamagnetic containment systems.

SUMMARY OF THE INVENTION The present invention relates generally to theproduction of high temperature plasmas and, more particularly, to thecontrol or elimination of instabilities from systems in which a hightemperature plasma is produced and confined in a magnetically definedzone.

Controlled fusion or controlled thermonuclear reactors with which thepresent invention may be practiced are those which include means forproducing a magnetic field in an evacuated region, said magnetic fieldhaving a configuration defining a zone in which charged particles, i.e.,a thermonuclear plasma, may be contained. Such reactors also include ameans for disposing or creating a high temperature plasma includingfusionable light isotope ions therein. Such light isotopes willgenerally comprise one or more materials such as one of the heavierhydrogen isotopes, i.e., deuterium alone or in admixture with tritium,helium 3 and possibly lithium which undergo fusion reactions underappropriate conditions of time, density and temperature.

The plasma may be formed by directing high energy particle beams such asenergetic molecular ion or neutral particle beams into the field to beionized by collisions, Lorentz force mechanisms, etc., yielding ionicparticles of higher charge to mass ratios which are trapped in thecontainment zone together with neutralizing electrons. Plasma may alsobe formed by directing energetic charged particles into the magneticfield and manipulating the field, e.g., by increasing magnetic fieldstrength, so as to trap the particles. Plasma may also be formed, insitu, e.g., by interaction of a neutral gas with energetic chargedparticles to ionize and heat the particles in the containment zone. Ingeneral, in producing such plasma, energetic atomic or molecularparticles, as well as energetic charged molecular, atomic or electronparticles, may be injected in the course of forming such a plasma. Theonset of instability may occur early in the formation of the plasma,i.e., during plasma density buildup to pose a serious or insurmountableobstacle to attaining desired density and/or period of containment.instabilities may also occur at later times to cause serious or totalplasma lossso that adequate containment times at a sustained reactionrate cannot be achieved, for example, in a controlled fusion reactor.

It is contemplated that the invention may generally be utilized with anysystem in which a high temperature plasma is confined in a containmentzone defined by a magnetic field. Moreover, it is contemplated that theteachings of the invention may be utilized for controlling a widevariety of instabilities. While a large number of apparently differenttypes of instabilities have been considered theoretically and/orobserved experimentally, as reported in the literature, it may beobserved that a large proportion, if not all, have a commoncharacteristic which may be utilized to effect control. This commoncharacteristic is related to the circumstance that localized oscillatorychanges in the density and/or position of localized portions of theplasma are associated with the occurrence of the instability. When theinstability attains a sufficient amplitude the plasma may locallypenetrate the field or the plasma otherwise may escape from thecontainment zone. Therefore, dependent on the stage at which plasma lossoccurs, an adequate density, temperature or time of containment cannotbe achieved.

The characteristic oscillatory changes of plasma and/or position of theplasma produce electromagnetic and electrostatic effects which may bedetected utilizing one or more electromagnetic, i.e., a pickup coil,detector probes or an electrostatic pickup detector probe mounted inproximity to the plasma boundary. The detector probes yield a signalthat is correlated with the frequency and amplitude of the instability.

For purposes of the invention the detected signal is amplified andshifted in phase and is then employed as a feedback signal or powersource to modulate an energetic particle beam. The modulated energeticparticle beam is then directed into the plasma containment zonegenerally to be trapped and contained therein as a plasma. The energeticcharged particle beam which is modulated may be derived from a sourceordinarily employed to form the plasma, i.e., as an ion or plasma sourceor a separate source may be employed particularly in a case where thereactor or device does not ordinarily employ an energetic particle beamsource in producing the contained plasma. In the latter case an electronbeam source may be used in the same manner. An energetic particle beamsource generally includes an ion source in which extracting electrodesdraw energetic charged particles from, e.g., an arc plasma region andmay include additional accelerating electrodes and focusing electrodesor magnetic focusing means for obtaining an energetic charged particlebeam. Such an energetic charged particle beam source may be employed todirect charged particles directly into a magnetic field plasmacontainment system. However, for many applications the energetic chargedparticles are passed through a neutralizer cell wherein the ionsinteract with a gaseous medium and acquire an electron becoming anenergetic neutral particle which is then directed into the plasmacontainment zone to be ionized and trapped therein to form or augmentthe plasma density. Energetic molecular ions may also be produced bysimilar means well known in the art to be injected and trapped in theplasma containment zone. With any such energetic particle beam source,the amplified feedback signal shifted in phase may be applied, forexample, to an accelerating or extraction electrode therein to yield aparticle beam current modulated in amplitude and/or energy with afrequency component corresponding to the feedback signal. Alternatively,the feedback signal may be directed through a modulation coil disposedcoaxially about a portion of the charged particle beam path, in such anenergetic particle source. to modulate the charged particle beamcurrent. It will be appreciated that the modulated component will beretained subsequent to neutralization so that modulation will be presentin the energetic particle beam whether the beam is composed of chargedparticles. energetic neutrals or energetic molecular ions.

The energetic particle beam may comprise particles of fusionable atomicspecies, i.e., ions, molecular ions or neutral particles particularlywhen a fusion plasma is to be stabilized, e.g., D", T*, Hey, DH", DT*,D, D DT, etc. The beam might also comprise energetic hydrogen ions,molecular ions or neutral particles, i.e., Hfl H or H and/or electrons.With plasmas intended for purposes other than fusion, the energeticparticles may comprise other atomic species and electrons compatiblewith such a plasma and the use intended.

Upon entry and trapping of the energetic particle beam in the plasmacontainment zone, the modulation component energy of the beam iscommunicated to the plasma in such a manner that oscillations areinduced in the contained plasma. The frequency and amplitude of theinduced oscillations are, of course, correlated to those of themodulation beam component and in the absence of the described phaseshift means could tend to be regenerative so as to reinforce theoriginal plasma instability. However, in accord with the concept of theinvention said phase shifter may be adjusted or arranged to cause themodulation component to be negatively out of phase' with the instabilityso that the amplitude of the instability is opposed and therebyminimized or eliminated. In a somewhat similar fashion the phase shiftmay be made positively out of phase thereby causing the plasmaoscillations or instability to be displaced in a manner in which thedestructive instability mode is diminished or eliminated. It iscontemplated that inphase regeneration could also be used to eliminate adestructive instability, for example, by selecting and reinforcing astable oscillation mode to predominate over an unstable or destructiveoscillatory mode.

As employed herein the terminology destructive or unstable oscillationor instability mode is intended to indicate a mode in which theoscillation or instability continues to grow in amplitude to a level atwhich the plasma is allowed or is caused to penetrate the magneticcontainment field and be lost. A stable mode may be represented by aquiescent plasma or by a plasma with an oscillatory mode or amplitude ofoscillation at which the plasma penetrates the containment field muchmore slowly, i.e., at times approaching the classical diffusion rate.

Accordingly, it is a general object of the invention to pro vide for theimproved confinement of a high temperature plasma in a magneticcontainment field.

Another object of the invention is to provide for the stabilization of amagnetically contained plasma by detecting plasma oscillationsorinstabilities and utilizing the detected signal to apply a modulationcomponent to a beam of energetic particles which is directed into thecontainment zone in such a manner that the modulation component of thebeam stabilizes the plasma containment.

Still another object of the invention is to provide for thestabilization of a magnetically contained high temperature plasma byutilizing at least one detector to produce a signal indicative of theplasma instability, shifting the phase of the detected signal andutilizing the phase shifted signal to modulate an energetic particlebeam which is directed into the plasma containment'field.

Other objects and advantageous features of the invention will beapparent in the following description and accompanying drawing in which:

FIG. 1 is a schematic representation of apparatus arranged in accordancewith the invention and employing an electromagnetic beam modulator; and

FIG. 2 is an illustration of an ion source as shown in H6. 1 which ismodified to permit electrostatic modulation of the ion beam.

As noted above the teachings of the invention may be employed for thestabilization of plasma containment in substantially any controlledfusion or thermonuclear reactor particularly where extended periods ofcontainment are desired. Among the suitable types of reactors are thosehaving linear, toroidal and generally spheroidal magnetic fieldconfigurations, e.g., Magnetic Mirror, Stellarator, Levitron, BaseballSeam and other types of reactors. Typical practice of the invention willbe described with reference to an ALlCE" type magnetic mirror reactordescribed, inter alia, in Plasma Physics and Thermonuclear Research, pp.55, et. seq., Volume 2, Progress in Nuclear Energy, Series Xl, PergamonPress, 1963.

Such an ALICE reactor, as illustrated in FIG. 1 of the drawing,generally includes a magnetically permeable metallic cylindrical vacuumvessel section 10 closed by end cover plates 11, 12, attached to flangesI3, 14, respectively, thereon and defining a chamber 16 therein. Asolenoid having terminal portions 17, 18 of a relatively high ampereturns/unit length characteristic and usually with a linear centralportion 19 of a unifonn relatively lower ampere turns/unit lengthcharacteristic is disposed circumjacent vessel section 10. Whenenergized with direct current from a supply (not shown) the solenoidproduces an axially symmetric magnetic field having intensified terminalregions 21, 22 and a central reduced intensity region therebetweendefining a magnetic mirror charged particle containment zone 23 withinchamber 16. The reactor also includes a means for directing a beam ofenergetic neutral particles into said containment zone. Such a meansgenerally includes an ion source 26 which produces a beam 27 ofenergetic ions, e.g., H, D*, T, Hef, etc. The ion source 26 may be ofthe Duo Plasmatron type described in aforesaid Plasma Physics andThermonuclear Research reference, cited above, or an equivalent device.In accord with usual practice the ion beam is directed through a gascell neutralizer 28 such as those disclosed in US. Pat. No. 3,152,959,issued Oct. 13, l964 to Charles'CJDamm. Therein the energetic ionsundergo charge exchange with a gaseous medium introduced through aconduit 29 and are converted into energetic neutral particles, i.e., H,D, T, He, etc. The energetic neutral particles emergent from theneutralizer are directed through beam tube section 31 connected to anaxial port in cover plate 11. Conduit section 31 is evacuated by meansof at least one vacuum pump (not shown) connected to a conduit sideann32 of beam tube 31. Neutralizing medium and other extraneous materialsare removed by said pump to establish an appropriate differential vacuumpressure, e.g., 10 mm. Hg. or lower, in the beam tube. A vacuum gatevalve 33 may be disposed in the beam tube 31 for closing off the beamsource.

The energetic neutral particles emerge from beam tube 31 as a collimatedbeam 34 which may be directed axially through magnetic mirror region 21into containment zone 23. The energetic neutral particles may be ionizedby collision with residual gas particles therein to form energetic ionswhich are trapped with electrons to form high temperature plasma 35confined in containment zone 23. It will be appreciated that theenergetic neutral particle beam may also be directed angularly ortransversely into the containment zone and that ionization can beeffected by Lorentz force particularly if the energetic neutrals are inan excited quantum state.

In usual practice, residual portions 36 of the energetic ion beam 34which traverse the containment zone 23 are directed through a conduit 37connected to an axial port in end cover plate 12. Vacuum pumpingequipment (not shown) and beam dumping or disposal means (not shown) maybe connected to conduit 37 to evacuate extraneous material from chamber16 and to dispose of residual beam particles. Such vacuum pump ingequipment together with other pumps (not shown) connected. if necessary,to the vessel evacuate chamber 16 to below about mm. Hg or preferablybelow about 10' to 10' mm. Hg.

For adapting a controlled fusion reactor, of the character described foroperation in accord with the invention, at least one probe means isdisposed in chamber 16 in a position of the plasma attendant to a plasmainstability or plasma oscillation. For example. at least oneelectrostatic probe may be so employed. Such probe may comprise a diskelectrode 41 supported in spaced proximity to a radial boundary ofplasma 35, e.g., by means of central conductor rod 42 of a standoffinsulator 43 leading in sealed relation through vessel section 10. Thepositioning of such a probe would be characteristic of those to be usedwith toroidal or cylindrical magnetic containment field geometries andrepresents a positioning of the electrode in a locale of unfavorablefield line curvature, i.e., concave toward the plasma at which plasmapenetration of the magnetic field and loss of the plasma due toinstability is most likely. With spheroidal or other geometries, similarcriteria may be used for appropriately positioning the probe means. Acomparable electromagnetic probe may-be provided by substituting a smallinsulated conductor coil (not shown) for the disk electrode 41. Probesof the foregoing types are described in Controlled ThermonuclearReactions," Glasstone Lovberg, D. Van Nostrand Co., Inc., Sections6.82-6.125.

in any event, such a probe delivers an electrical signal indicative ofthe frequency and amplitude of an instability oscillation to conductor44. The frequency of an instability oscillation may range from about afew cycles per second up to the harmonics of the ion cyclotron frequencyof plasma particles in the magnetic field, i.e., up to about 10megacycles/second. Typical low frequency oscillation would lie in therange of about 10 cycles to 100 kilocycles while high frequency modeswould lie in a range of about 100 kilocycles to ID megacycles. Thesignal from conductor 44 is fed into a phase shifter 46, wherein thesignal may be amplified if necessary and the phase of the instabilitysignal is generally shifted either to lead or to lag in phase relationwith respect to the phasing of the instability oscillation. For maximumfeedback control the phase shift should be of the order of 180 in eithera leading or lagging relation. However, effective phasing may beobtained with a phase shift effectively of the order of about 45 toabout 315. While it is generally preferred to employ a phase shift inthe detected signal for generating effectively degenerative feedbackpower utilized, as described hereinafter, to control an instability, anin-phase signal might be used regeneratively, for example. to reinforcea stable oscillation mode to predominate over and thereby exclude anunstable mode of oscillation. An appropriate filter network (not shown)or the like may be utilized to select the desired regenerative signal aswell as to assure selection of a desired instability signal whennecessary.

The degenerative phase shifted signal or the regenerative signal ofselected frequency is fed by means of conductor 47 into a poweramplifier. The amplified signal therefrom is then employed to modulatethe ion beam 27 produced by source 26. An electromagnetic means formodulating the ion beam is illustrated in FIG. 1. A flanged beam tubesection 51 is disposed between ion source 26 and neutralizer 28 so thation beam passes therethrough. A solenoidal coupling coil 52 is supportedcircumjacent ion beam 27 as by means of a feedthrough standoff insulator53 mounted in sealed relation on beam tube section 51. The modulationcurrent from amplifier 48 is delivered to coil 52 by means of a pairedtransmission line 54 and thereby impress a modulation envelope on theion beam 27 which is preserved in the energetic neutral particle beam 34during passage through the neutralizer 28. The modulated component ofenergy is transferred to the plasma 35 when the energetic neutralparticles are trapped in containment zone 23 wherein a degenerativeeffect is induced effective to decrease the amplitude of a destructiveinstability. A regenerative effect is produced in the second case so asto cause a stable oscillation mode to predominate to the exclusion of anunstable mode.

Electrostatic means may also be employed to modulate ion beam 27. lnthis case the electromagnetic modulation means shown in FIG. 1 isomitted and'the ion source 26 may be modified as shown in FIG. 2. Moreparticularly, a cylindrical tubular electrode 52 may be mountedconcentrically about the path of ion beam 27 within the exterior tubularcasing 57 of such ion source 26 as by means of feedthrough standoffinsulator 58 supported in sealed relation by casing 57. The centralconductor 59 of standoff insulator 58 is then connected to one leg oftransmission line 54 and the'other leg of transmission line 54 isconnected to a suitable reference voltage point of ionsource 26, notshown. Application of the modulation power from amplifier 48 thenelectrostatically impressed the feedback modulation to ion beam 27 andthe ion beam thenceforth behaves as described above. it would, ofcourse, be possible to arrange the electrostatic modulation electrode asin the case of the electromagnetic unit. Moreover, the modulation powermight also be superimposed on the'accelerating voltage applied to anextractor or acceleration electrode already existent in the ion source.

it may be noted that controlled fusion reactors utilize magnetic fieldsranging from a few hundred gauss, for laboratory devices, to above about200 kilogauss in intensity'for power producing designs. ion beamenergies mayrange from as low as l KEV. to above about I00 KEV.Laboratory devices may be operated with charged particle (plasma)densities as low as l0 particles/cc. However, significant neutron andpower production generally require densities of the order of 10 to aboveabout l0 particles/cc, i.e., D", T, He and mixtures thereof withcontainment times of the order of at least 0.] second and attemperatures ranging from about 5 KEV. to above 100 KEV.

The present invention may be particularly useful during buildup of theplasma density since a wide variety of density, magnetic field strength,and/or temperature conditions may be progressively encountered duringthis phase of operation which are conducive to the stagewise generationof instabilities.

While the stabilizing feedback modulation energy has been applied to theion beam utilized to produce the plasma in the foregoing embodiment, itwill be appreciated that it will be ap propriate in other circumstancesto employ such a system as an accessory or to utilize an electron beamsource in a similar manner. Such a circumstance would occur, forexample, where a high temperature plasma is formed, in situ, withoututilizing an ion source as may be the case in certain toroidalgeometries.

Further details of the invention will be set forth in the followingillustrative example.

EXAMPLE A typical laboratory scale ALICE" machine with which theinvention may be practiced may have the following parameters:

Fusion fuel deuterium Final plasma density 10" to to" particles cm."

ion temperature 20-40 KEV. Length containment zone 100 cm. Radiuscontainment zone 50 cm. Plasma radius l2 cm.

Central Magnetic field Magnetic mirror fields Magnetic mirror ratioTypical high-frequency unstable oscillation mode Typical low-frequencyinstability oscillation mode Feedback phase shift 2 to 270 5-25kilogauss B-40 kilogauss 8 megacycles/sec.

20 kilocycles tended to cover all such modifications as fall within thescope of the appended claims.

What we claim is:

1. ln apparatus for producing and containing a high temperature plasma,the combination comprising:

means for producing a magnetic field defining a high temperature plasmacontainment zone in an evacuated region;

means for creating a high temperature plasma in said con tainment zone,said means including at least one source for directing a beam ofenergetic particles into said containment zone;

detector probe means disposed proximate said containment zone fordetecting localized oscillatory variations in the density and positionof the plasma in said containment zone and for producing a signalcorrelated with said oscillatory variations;

means for amplifying said signal; and

means coupled to said amplifying means for coupling the amplified signalto modulate said beam of energetic particles which are directed intosaid containment zone so that the modulation of said beam stabilizescontainment of said plasma in the containment zone defined by saidmagnetic field.

2. Apparatus as defined in claim I wherein at least said one source fordirecting a beam of energetic particles into said containment zone is acharged particle source and which creates a beam of energetic chargedparticles which is directed into said containment zone.

3. Apparatus as defined in claim 1 wherein at least said one source fordirecting a beam of energetic particles into the containment zoneincludes an ion source in combination with a neutralizer through which abeam of ions from said ion source is directed to be neutralized to forma beam of energetic neutral particles which is directed into saidcontainment zone to be ionized and trapped therein.

4. Apparatus as defined in claim 3 wherein there is included phaseshifting means operatively coupled to said probe means and delivering asignal shifted in phase to said amplifying means so that a degenerativemodulation component is applied to said beam of energetic neutralparticles for opposing said localized oscillatory variations in theplasma density and position.

5. Apparatus as defined in claim 4 wherein the phase shift provided bysaid phase shifting means is in the range of about 45 to about 3 l5 outof phase with said oscillatory variations.

6. Apparatus as defined in claim 4 wherein the output of said signalamplifying means is applied to a solenoidal coupling coil arrangedconcentrically about the path of said ion beam for modulating said beam.

7. Apparatus as defined in claim 4 wherein the output of said signalamplifying means is applied to a cylindrical tubular electrode mountedconcentrically about the path of said ion beam for modulating said beam.

8. Apparatus as defined in claim 6 wherein said phase shifting meansshifts the phase of said signal to the range of to 270 out of phase withrespect to said oscillatory variations.

9. Apparatus as defined in claim 7 wherein said phase shifting meansshifts the phase of said signal to the range of 90 to 270 out of phasewith respect to said oscillatory variations.

10. Apparatus as defined in claim 8 wherein said magnetic field includesa locale of unfavorable line curvatures at which plasma penetration ismore likely and wherein said probe means is disposed in proximity tosaid locale.

2. Apparatus as defined in claim 1 wherein at least said one source fordirecting a beam of energetic particles into said containment zone is acharged particle source and which creates a beam of energetic chargedparticles which is directed into said containment zone.
 3. Apparatus asdefined in claim 1 wherein at least said one source for directing a beamof energetic particles into the containment zone includes an ion sourcein combination with a neutralizer through which a beam of ions from saidion source is directed to be neutralized to form a beam of energeticneutral particles which is directed into said containment zone to beionized and trapped therein.
 4. Apparatus as defined in claim 3 whereinthere is included phase shifting means operatively coupled to said probemeans and delivering a signal shifted in phase to said amplifying meansso that a degenerative modulation component is applied to said beam ofenergetic neutral particles for opposing said localized oscillatoryvariations in the plasma density and position.
 5. Apparatus as definedin claim 4 wherein the phase shift provided by said phase shifting meansis in the range of about 45* to about 315* out of phase with saidoscillatory variations.
 6. Apparatus as defined in claim 4 wherein theoutput of said signal amplifying means is applied to a solenoidalcoupling coil arranged concentrically about the path of said ion beamfor modulating said beam.
 7. Apparatus as defined in claim 4 wherein theoutput of said signal amplifying means is applied to a cylindricaltubular electrode mounted concentrically about the path of said ion beamfor modulating said beam.
 8. Apparatus as defined in claim 6 whereinsaid phase shifting means shifts the phase of said signal to the rangeof 90* to 270* out of phase with respect to said oscillatory variations.9. Apparatus as defined in claim 7 wherein said phase shifting meansshifts the phase of said signal to the range of 90* to 270* out of phasewith respect to said oscillatory variations.
 10. Apparatus as defined inclaim 8 wherein said magnetic field includes a locale of unfavorableline curvatures at which plasma penetration is more likely and whereinsaid probe means is disposed in proximity to said locale.